The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods with output detection and synchronized rectifying mechanisms. Merely by way of example, the invention has been applied to a power conversion system. But it would be recognized that the invention has a much broader range of applicability.
FIG. 1 is a simplified diagram showing a conventional flyback power conversion system. The power conversion system 100 includes a primary winding 110, a secondary winding 112, a power switch 120, a current sensing resistor 122, a rectifying diode 124, a capacitor 126, an isolated feedback component 128, and a controller 102. The controller 102 includes an under-voltage-lockout component 104, a pulse-width-modulation generator 106, a gate driver 108, a leading-edge-blanking (LEB) component 116, and an over-current-protection (OCP) component 114. For example, the power switch 120 is a bipolar transistor. In another example, the power switch 120 is a field effect transistor.
The power conversion system 100 implements a transformer including the primary winding 110 and the secondary winding 112 to isolate an AC input voltage 190 on the primary side and an output voltage 192 on the secondary side. The isolated feedback component 128 processes information related to the output voltage 192 and generates a feedback signal 136. The controller 102 receives the feedback signal 136, and generates a gate-drive signal 130 to turn on and off the switch 120 in order to regulate the output voltage 192. For example, the isolated feedback component 128 includes an error amplifier, a compensation network, and an opto-coupler.
Though the fly-back power conversion system 100 can be used for output voltage regulation, the power conversion system 100 often cannot achieve good output current control without additional circuitry of high cost. Moreover, the required output current sensing resistor in the secondary side usually reduces the efficiency of the power conversion system 100.
FIG. 2(A) is a simplified diagram showing another conventional flyback power conversion system. The power conversion system 200 includes a system controller 202, a primary winding 210, a secondary winding 212, an auxiliary winding 214, a power switch 220, a current sensing resistor 230, two rectifying diodes 260 and 262, two capacitors 264 and 266, and two resistors 268 and 270. For example, the power switch 220 is a bipolar transistor. In another example, the power switch 220 is a MOS transistor.
Information related to the output voltage 250 can be extracted through the auxiliary winding 214 in order to regulate the output voltage 250. When the power switch 220 is closed (e.g., on), the energy is stored in the transformer that includes the primary winding 210 and the secondary winding 212. Then, when the power switch 220 is open (e.g., off), the stored energy is released to the secondary side, and the voltage of the auxiliary winding 214 maps the output voltage on the secondary side. The system controller 202 receives a current sensing signal 272 that indicates a primary current 276 flowing through the primary winding 210, and a feedback signal 274 that relates to a demagnetization process of the secondary side. For example, a switching period of the switch 220 includes an on-time period during which the switch 220 is closed (e.g., on) and an off-time period during which the switch 220 is open (e.g., off).
FIG. 2(B) is a simplified conventional timing diagram for the flyback power conversion system 200 that operates in the discontinuous conduction mode (DCM). The waveform 292 represents a voltage 254 of the auxiliary winding 214 as a function of time, and the waveform 294 represents a secondary current 278 that flows through the secondary winding 212 as a function of time.
For example, as shown in FIG. 2(B), a switching period, Ts of the switch 220, starts at time t0 and ends at time t3, an on-time period, Ton, starts at the time t0 and ends at time t1, a demagnetization period, Tdemag starts at the time t1 and ends at time t2, and an off-time period, Toff, starts at the time t1 and ends at the time t3. In another example, t0≦t1≦t2≦t3. In DCM, the off-time period, Toff, is much longer than the demagnetization period, Tdemag.
During the demagnetization period Tdemag, the switch 220 remains open, the primary current 276 keeps at a low value (e.g., approximately zero). The secondary current 278 decreases from a value 296 (e.g., at t1) as shown by the waveform 294. The demagnetization process ends at the time t2 when the secondary current 278 has a low value 298 (e.g., approximately zero). The secondary current 278 keeps at the value 298 for the rest of the switching period. A next switching period does not start until a period of time after the completion of the demagnetization process (e.g., at t3).
As shown in FIG. 1 and FIG. 2(A), the power conversion system 100 and the power conversion system 200 each use a rectifying diode (e.g., the diode 124 in FIG. 1 and the diode 260 in FIG. 2) on the secondary side for rectification. A forward voltage of the rectifying diode is usually in a range of 0.3V-0.8V. Such a forward voltage often results in significant power loss in operation, and thus causes low efficiency of the power conversion system. For example, when a power conversion system has an output level of 5V/1 A, a rectifying diode with a forward voltage of 0.3V-0.4V causes a power loss of about 0.3 W-0.4 W at a full load (e.g., 1 A). The reduction of the system efficiency is about 4%-6%.
In addition, in order for the power conversion system 200 to achieve low standby power consumption, the switching frequency is often kept low to reduce switching loss under no load or light load conditions. However, when the power conversion system 200 changes from no/light load conditions to full load conditions, the output voltage 250 may drop abruptly and such a voltage drop may not be detected by the system controller 202 instantly because the system controller 202 can often detect the output voltage only during a demagnetization process of each switching cycle. Therefore, the dynamic performance of the power conversion system 200 is often unsatisfactory at a low switching frequency under no/light load conditions. For example, the power conversion system 200 has an output level of 5V/1 A and the output capacitor 264 has a capacitance of 1000 μF. Under no/light load conditions, the switching frequency is 1 kHz corresponding to a switching period of 1 ms. If the output load changes from no/light load conditions (e.g., 0 A) to full load conditions (e.g., 1 A), the output voltage 250 drops 1 V (e.g., from 5 V to 4 V), which is often unacceptable in certain applications.
Hence, it is highly desirable to improve techniques for rectification and output detection of a power conversion system.